In recent years, acoustic emission testing has been used to measure how much a structure can be loaded before developing a crack, detecting cracks in structures, and detecting internal leaks in fluid-carrying equipment (e.g., valves). During acoustic emission testing, a structure undergoing testing is gradually loaded to known stresses (typically greater than a service stress the structure will experience in actual use) and acoustic emission sensors are used to detect cracks as they occur in the loaded structure (e.g., sizes and locations of the cracks). In other cases, a valve is monitored with an acoustic emission sensor to detect internal leaks as they occur within the valve. Acoustic emission sensors typically include a receiver coupled to a wear plate, connected to a computer via a data cable, and covered by a housing. The receiver typically includes a piezoelectric element or transducer. The wear plate typically acts as an interface (e.g., a shield) between the receiver and the structure undergoing testing. When an acoustic emission resulting from a structural change (e.g., production of a crack, production of an internal valve leak, etc.) arrives at the receiver, the piezoelectric transducer produces an electrical signal that is transmitted to the computer via the data cable.
However, acoustic emissions resulting from cracks are not repeatable (e.g., only one crack and one respective acoustic emission occur for a given test load). Therefore, the acoustic emission sensor must detect the acoustic emission at the singular occurrence when the structural change first develops to collect precise data for the structure undergoing acoustic emission testing. Further, while internal leaks continuously produce acoustic emissions, the acoustic emission sensor must precisely detect the acoustic emissions to determine the severity of the internal leak. Precise acoustic emission sensing is highly dependent on a good (e.g., acoustically transmissive) bond between the wear plate and the tested structure.
To ensure a good bond between the tested structure and the wear plate, a fluid (e.g., glue, grease) is typically used between the structure and the wear plate to fill voids (typically microscopic) between the wear plate and the structure that would otherwise impede acoustic transmission from the structure to the acoustic emission sensor. Further, the bond is tested for acoustic transmissibility by applying a test acoustic signal of a known intensity to the structure. Reception of the test signal by the receiver at or near the initial intensity is indicative of a good bond.